Electrical power supplied by public or private utilities is susceptible to transient voltage variations attributable to lightning strikes and/or switching malfunctions. Customers traditionally have borne the brunt of transient surges, and had their light bulbs and motor windings fail ("burn out") from time to time as a result. However, that is no longer acceptable in an age of solid-state electronic devices, which often may be damaged worse by smaller surges than their appliance motors or radios withstood in the past. The desirability of protecting utility meters and downstream electrical equipment from voltage surges is obvious, and customers expect to receive such protection. Indeed, regulatory agencies and the courts are imposing upon public utilities an obligation to assure suitability of product/service as delivered rather than resorting to a force majeure or "act of God" concept to excuse preventable damage to a customer's equipment or installation.
Accordingly, electrical utilities are now having to exercise more quality control over their product/service, to render it truly merchantable for modern-day uses--or be potentially liable, in the event of surge damage, for not doing so. As in other instances of readily available safety measures, insurance companies are able to foster adoption of transient voltage surge suppression, by providing more favorable rates where TVSS has been installed.
Conventional watt-hour meters are commonly connected between an external power source and electrical equipment downstream, so as to measure the amount of electrical energy or power utilized by the downstream equipment. Such meters have as contactors protruding bladelike terminals that plug into a socket connected to power-line leads at a utility box or panel, as at a power customer's location. As suggested by the present inventor in 1986, in the earliest of his aforementioned patent applications, the vicinity of the watt-hour meter is his preferred site for transient voltage surge suppression.
For many decades electrical utility personnel have known that switches and other circuit-interruption devices can be installed in a cylindrical housing, now known as a meter adapter, for plug-and-jack interposition between a watt-hour meter and its socket, as was pointed out in the present inventor's earliest patent application by reference to patents granted to St. John and Megarian, for example.
St. John in U.S. Pat. No. 2,606,232 (1952) disclosed interposable adapter means providing for circuit-interruption, by manual switch at will, and by circuit breaker and/or fuse in the event of current overload of such amount and duration as to provoke an interruption. St. John's objectives were to free authorized personnel from having to enter the premises serviced by the power lines in order to shut off the power and/or to replace fuses or service circuit breakers. St. John's arrangement left the meter connected to the power lines despite interruption of the downstream circuit for any reason, so it did not protect the meter itself against transients on power lines.
Megarian in U.S. Pat. No. 3,599,047 (1971) elaborated upon manual switching, including disconnecting the watt-hour meter from the line power when the downstream circuit was not to be enabled. When the line power was shut off, Megarian's arrangement protected the meter as well as the downstream electrical equipment--but both the meter and the downstream equipment remained susceptible to damage from a transient voltage surge received via the power lines when connected.
Neither St. John's nor Megarian's arrangement provided any TVSS protection, not only because obviously manual switches cannot do so, but also because over-current devices, such as fuses or circuit breakers, even when connected, are not actuated by fractional-second transient surges but only by current overloads of appreciably longer duration--and after actuation must be replaced or reset. As already noted, even St. John's fuses and circuit breakers were not connected in the circuit to the watt-hour meter so could not even protect it against a protracted current overload sufficient to actuate a fuse or a circuit breaker.
Protection against over-voltage, as distinct from protection against over-current conditions (such as a current overload), calls for completely different electrical means. The archetype of such means is a spark gap. Spark gaps have been used for years on power lines, but open-air spark gaps are erratic in their action and are unsafe for use at a customer's premises, whereas analogous gaps in gas-filled envelopes lack adequate response time to cope with the transient surges received from power lines.
A watt-hour meter has a predictably long life, even a quarter century if protected against excessive voltages and currents. Some workers in the art undertook to provide such meters with protection against damaging voltage transients as well as current overloads.
A spark gap undergoes no electrical conduction until a large enough voltage differential builds up across its terminals between power line and ground to cause an arc between them, whereupon the spark gap clips the voltage transient and conducts the resulting surge current to the external ground. The required voltage differential is a function of gap size, terminal shape, intervening gas(es), gas pressure, and humidity. The voltage drop across a conducting arc is much smaller than the voltage required to initiate or "strike" such arc. Once struck, an arc may become self-sustaining or at least difficult to extinguish, as current "follows on" through the resulting low-impedance gap filled with already ionized air. A standard remedy was insertion of a high-resistance, in series with the spark gap, as via a Nichrome wire. At the low voltage present at watt-hour locations, a high series resistance tended to prevent the spark gap from striking so as to clip intermediate-level voltage surges hazardous to meters and to shunt resulting current to ground.
Workers in the art attacked this problem by substituting a variable-resistance means for the fixed-resistance Nichrome wire in series with the spark gap. Zisa in U.S. Pat. No. 3,725,745 (1973), and Melanson in U.S. Pat. No. 3,914,657 (1975), connected solid-state variable-resistance devices between power lines and the spark gaps. The function of such a device (characterized by inverse relationship between its resistance and applied voltage) was to conduct readily whenever the voltage surged so high that the spark gap would conduct but to discontinue conducting at the lower follow-on voltage so as to extinguish the arc promptly upon cessation of the voltage surge. Although similar arrangements were adopted for power lines at higher voltages, they were not adopted at the usual watt-hour locations.
Other workers, such as Dell Orfano in U.S. Pat. No. 4,089,032 (1978), adopted "varistors" (a class of such solid-state devices) for over-voltage protective apparatus adapted to be plugged into electrical wall outlets upstream of whatever is to be operated by electricity drawn from such outlets. Such protective apparatus may contain additional components, such as glow tubes or diodes, but at least in the absence of such added components, the varistors operate to clip voltage surges at a given level above normal power voltage and shunt their surge currents away (usually to ground) before reaching the downstream equipment. In this regard a varistor selected to conduct minimally at normal power voltage has taken on the primary function (formerly performed by a spark gap) of clipping the surge above a preselected higher voltage, and a modified function of minimal conduction (substantial non-conduction) at power voltages.
Despite (or perhaps because of) the diversity of teachings in such patents, and notwithstanding the two decades between St. John's meter adapter and the aforementioned 1970's patents, no one then or in the decade of the 1980's provided a meter adapter with built-in TVSS until the present inventor first did so in the mid-1980's.
Despite the well recognized need for transient voltage surge protection, the resulting meter-based TVSS revolution, now under way was impossible to foresee. For whatever reasons, the art was stagnant, and the contributions of inventors noted above had not given rise to the present methods or apparatus. Once the present inventor had shown the way, entry into the field occurred so rapidly as to underscore the unlikelihood that the prior art would have provided such contributions to this surge-suppression art unaided.
The steps of installing a TVSS meter adapter could hardly be much simpler. The installer first unplugs the watt-hour meter from its socket in the customary utility box or panel, then plugs the adapter (in place of the meter) into the socket, and finally plugs the meter into the meter adapter as the meter had been previously plugged into the socket. Installation requires only a few minutes.
Producing an effective TVSS meter adapter is quite difficult because of the extreme conditions imposed upon the apparatus by the transient surges. In view of the very high, but quite short, voltage surges and the very large resulting surge currents, testing sites for apparatus of this general type have been severely limited (and testing costly) so that in most instances it is impracticable for users to test competing equipment or to verify (or disprove) operating specifications or technical claims of suppliers. The present inventor constructed a lightning simulation laboratory, with computerized documentation of the duration and height of voltage surges to thousands of volts and the intensity of surge currents to tens of thousands of amperes.
Competing manufacturers, whether they have or have not actually tested their products under surge voltages and resulting currents, overwhelmingly have adopted the meter adapter as the preferred means of housing varistors connected to provide TVSS at the watt-hour meter location. Competitors also have included, with noteworthy alacrity, various improvements--whether patented or not--provided by the present inventor, who has pioneered increased surge capacity and safety of surge-protective apparatus.
Examples of his inventions include ground-plane heat-sinking of component varistors (see his aforementioned patent); inserting temperature-responsive or "thermal" fuses or like cutoff devices to sense temperature and to disconnect the varistors from the power lines to preclude failure from excessive temperature rise (as in U.S. Pat. No. 4,866,560); stacking varistor disks in parallel circuit (in U.S. Pat. Nos. 4,901,187, 5,006,950, and 5,148,345); and improved plug-and-jack connectors (as in U.S. Pat. Nos. 4,944,692 and 5,129,841). Other examples of his TVSS contributions have focused upon a hard-wired unit, including distributed-resistance fuse links, with or without thermal cutoff means and/or varistor stacking, between power line leads and varistors (as in U.S. Pat. Nos. 4,907,119 and 5,148,345); and arc extinguishing (as in U.S. Pat. No. 5,140,491).
The present application is directed to the work product of his early insights, especially as embodied in a meter adapter. His article "Facility Surge Protection" in the initial (1990) issue of POWER QUALITY Magazine (p. 47) notes their technical superiority, as do third-party papers, including (i) Power Quality Association PQA-91 Report B-22- Residential Transient Voltage Surge Suppression Program, by A. Michael Maher of Potomac Electric Power Company (Pepco), Washington, D.C., detailing demand for and utility of TVSS meter adapters, along with results of comparative testing of several brands of such adapters (Brand D is the present inventor's adapter, the unit with the largest diameter varistors--thus avoiding any need for dubious parallel interconnection of smaller varistors as in other competitors' units); (ii) Residential Service Entrance Surge Suppression Device Testing & Considerations by Raymond C. Hill of Georgia Power Company, which details its own testing rationale and program and has an extensive autobiography (the unit finally selected at page 11, .paragraph.3 was the present inventor's); and (iii) March 1992 CEE News article "Power Quality Pays Off" by J. David Lankutis, about the success of the present inventor's meter adapters installed by San Miguel Power Association in coping with a given surge event.